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2.8.1.2.2 Using Chiral Iridium Catalysts with Allyl Acetates

DOI: 10.1055/sos-SD-202-00187

Buitrago Santanilla, A.; Leighton, J. L.Science of Synthesis: Stereoselective Synthesis, (20112414.

An interesting approach to enantioselective aldehyde allylation has been developed based on the reductive and catalytic generation of allylmetal species from a simple and inexpensive allyl source such as allyl acetate. One key innovation at the heart of this work is the use of alcohols as the reductant where stoichiometric amounts of metal-based reductants had been used previously.[‌29‌‌31‌] Although this has resulted in the development of highly enantioselective aldehyde allylation[‌32‌] and anti crotylation[‌33‌] reactions (using propan-2-ol as the reductant) it must be noted that the reactions suffer from significant practical liabilities that call into question whether this method will find widespread adoption as a more practical alternative to some of the methods discussed above, especially for larger-scale applications: (1) the iridium catalyst and some of the ligands employed are relatively expensive and the use of a heavy metal catalyst in and of itself carries with it some significant disadvantages, (2) the reactions are conducted in a sealed pressure tube in tetrahydrofuran at 90 or 100°C, and in addition to safety concerns this poor reactivity apparently limits the scope of the crotylation reaction to relatively unhindered aldehydes, and (3) they are type III reactions, and therefore can only provide access to the anti crotylation products, and then only with moderately high (typically 6:1 to 11:1) levels of diastereoselectivity.

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M Meeeee ee ee (%) Meeee (%) Mee
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MMMe (M)-88 88:8 88 88 [‌88‌]
MMe8 (M)-88 88:8 88 88e,e [‌88‌]
(M)-88 88:8 88 88e [‌88‌]
M=MM8 (M)-88 88:8 88 88 [‌88‌]
(MM8)8 (M)-88 88:8 88 88 [‌88‌]
(MM8)8 (M)-88 88:8 88 88 [‌88‌]

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